1. Technical Field
The present invention relates to a novel use of partly prehydrated lime, which contains CaO that reacts with H2O after a certain lag phase, that is to say a delayed reactivity quicklime (abbreviated to: “DRQL”), in the field of separating solid matter from a mixture comprising said solids and a liquid (i.e. liquid/solid separation, denoted hereinafter for convenience by “solid matter/liquid separation” or “solids/liquid separation”), and in particular in the field of separating the solid matter from a suspension in a liquid. It also relates to a method for treating a sludge, in which the solids are concentrated, decontaminated, dried and recovered using said delayed reactivity partly prehydrated lime. Finally it relates, as novel industrial products, to (a) the purified sludge (denoted hereinafter by “solid material derived from a sludge”) obtained according to this method, on the one hand, and to (b) the combination of a DRQL with at least one Fe3+ and/or Al3+ salt, which is used to carry out this method, on the other hand.
2. Discussion of the Prior Art
It is known that various techniques for solids/liquid separation have been developed in order to concentrate the solid matter contained in a solids/liquid mixture when at least one portion of said solids is dispersed in the liquid of said mixture, especially when they are in the form of suspensions of particles, such as is the case for sludges.
Said concentrated mixtures thus obtained, especially the concentrated suspensions, must often undergo subsequent treatments before being able to be handled, stored or reused. These treatments use reactants whose role is, for example, to modify the chemical properties, to decontaminate (i.e. to sanitize or clean up) or else to modify the physical properties of the concentrated solid matter/liquid mixture or of the concentrated suspension, such as the rheology or the plasticity.
It is known that the final solids concentration of a solids/liquid mixture or of a suspension is an important factor that should be optimized in order to promote the implementation of the treatment, handling and storage steps. From this viewpoint, it has been proposed to improve the performances of the solid matter/liquid separation methods, for example:                on the one hand, by means of a chemical treatment of the solid matter/liquid mixtures or of the suspensions resulting in precipitation and flocculation phenomena; and        on the other hand, by optimizing the equipment itself, the term “equipment” here denoting the solids/liquid separation conditions and devices [in particular the centrifuges and filtration equipment such as filter presses (especially plate or tube filter presses) and belt filters (especially pressurized or vacuum belt filters)].        
It is also known that lime may be used to improve the performance of the solid matter/liquid separation methods. It is in that case added to the suspension before the solid matter/liquid separation (in this case a person skilled in the art refers to this as “pre-liming”, i.e. a liming operation before filtration, centrifugation or settling) or after said separation (in that case a person skilled in the art refers to this as “post-liming”, i.e. a liming operation after filtration, centrifugation or settling).
Pre-Liming
Within the context of pre-liming, it is found that quicklime is not suitable when the solids/liquid mixture or suspension contains a liquid that reacts with it, such as water. This is because the violent reaction between quicklime and water (i) results in unhomogeneous mixtures, and (ii) disturbs the solid matter/liquid separation. Therefore it is preferred to introduce hydrated lime Ca(OH)2 either in powder form or in the form of an aqueous suspension, into said mixture or said suspension to be concentrated. The Ca(OH)2 particles, thus added or formed by reaction of the lime with the water from the solid matter/liquid mixture or from the suspension, have a size typically of less than 10 μm; these particles are too small to be able to increase the porosity of the separation residue or cake and thus to promote the desiccation and decontamination of the solid matter/liquid separation cake.
Even if the use of quicklime added in the form of larger particles is able to promote the centrifugation of the sludges by increasing the porosity of the filtration or centrifugation cake (as described in the article published in Wat. Sci. Tech., 1993, Vol. 28 (No. 1), pp 223-231], the problem of the violent reaction between the lime and the water from the solid matter/liquid mixture or from the suspension, which is not resolved, remains unacceptable.
It is known from the article by S. DENNEUX-MUSTIN et al., Wat. Res., 2001, vol. 35 (No. 12), pp 3018-3024, that the use of lime combined with FeCl3 promotes the mechanical dehydration of the sludges. This effect is also observed when the lime is added alone to a sludge that already contains iron as described in WO 99/10288 A.
It is also known that, during the use of organic polymer flocculants, the addition of lime generates a high pH that causes, in most cases, their inactivation or destruction by alkaline hydrolysis. The granted patent EP 1 154 958 B proposes to get round this difficulty by controlling or delaying the increase of the pH; to that effect, the recommended solution comprises the addition, to the sludge to be concentrated, of an organic flocculant and a particular lime within the context of pre-liming, said lime being chosen from the group formed (according to the wording of claim 1 of said patent) by:                quicklime;        hydrated lime in powder form;        hydrated lime in suspension in water, having a particle size d50≧50 μm;        overburnt lime;        quicklime containing a fluid additive having an ability to agglomerate fine particles;        quicklime containing a hydration-retarding agent;        hydrated lime having elementary agglomerated particles;        hydrated lime to which an agent has been added for slowing down the activity [i.e., according to the language used by EP 1 154 958 B in paragraph [0019], a product that slows down the dissolution of Ca(OH)2 in water];        hydrated lime having a solids content greater than 20% by weight; and        “unfilled” lime [i.e. a lime from which the fine particles have been removed, in particular those having a size of less than or equal to 200 μm].        
The description of EP 1 154 958 B points out, in addition, the possibility of requiring a DRQL for the pre-liming operation (see, to that effect, the last sentence of paragraph [0026] of this patent) in order to slow down the slaking reaction. However this patent neither describes nor suggests the feature of the present invention, mainly that said DRQL intervenes in the agglomeration of solids during the pre-liming operation; then in the post-liming operation for the desiccation and decontamination of the solids contained in the sludges.
Post-Liming
Furthermore, the use of quicklime, during the post-liming of the concentrated solid matter/liquid mixtures or concentrated suspensions each containing water, certainly has numerous advantages. It is known, in particular, from WO 02/32818 A that the increase of the pH and of the temperature associated with the reaction of the quicklime with the residual water contained in the wet separation cakes ensures their sanitization. This is because, besides the fact of generating heat, it is found that the reaction (1) between water and quicklime:CaO+H2O→Ca(OH)2  (1)consumes the water and makes it possible to increase the level of dryness of the separation cake, as indicated in U.S. Pat. No. 4,279,279 A, the quicklime CaO actually being capable of capturing around 32% of its weight in water.
However, as in the case of the pre-liming, it is also difficult to obtain in post-liming a homogeneous mixture due to the violence of the aforementioned reaction (1). In addition, the post-liming requires an additional step, namely the mixing of said concentrated solid matter/liquid mixture, or of said concentrated suspension, with the quicklime. This mixing step has the drawback of increasing the cost and the complexity of the operating conditions, on the one hand, and of disintegrating said concentrated solid matter/liquid mixture or said concentrated suspension, forming the separation cake, by the mechanical stresses that it generates, on the other hand.
The drawback linked to the increase in the number of steps for treating the separation cake is found in U.S. Pat. No. 5,679,262 A, which describes a post-liming technique consisting of two steps. This technique certainly enables the quantity of quicklime required for treating a sludge to be reduced by 40%, but has the disadvantage of making the processing conditions of said treatment more complex.
DRQL
Furthermore, delayed reactivity partly prehydrated quicklimes (DRQLs) are known. These products are formed from CaO and from Ca(OH)2. They are in the form of a mixture of quicklime CaO particles and hydrated lime Ca(OH)2 particles, arranged in such a way that, in the presence of water, the aforementioned reaction (1) is delayed. In the current state of knowledge, there is every reason to presume that the DRQL is in the form of quicklime CaO particles, forming a core, and of hydrated lime Ca(OH)2 particles, forming a temporarily protective layer or film covering the CaO core. Even though this is a theory that does not tie the Applicant, the description “CaO core covered with a temporarily protective Ca(OH)2 membrane or layer” is particularly practical for understanding the delay mechanism of the reaction (1). On this subject, the granted French application FR 2 841 895 A proposes, in particular, a DRQL that can be used, as a binding agent for compacting a mineral filler (such as clay, earth, cement, sludges and mixtures thereof) in the manufacture of shaped articles. According to the teaching of FR 2 841 895 A, said DRQL is involved, in the manufacture of said shaped articles, in the form of a powder or, thanks to the addition of a plasticizer (denoted “superplasticizer” by a person skilled in the art), in the form of a fluid aqueous suspension and, where appropriate, it is concentrated.
Compacting
Compacting (especially according to JP 60/054799 A or the aforementioned FR 2 841 895 A) of a waste, such as a sludge, should not be confused with the pre-liming operation or the post-liming operation. Compacting of a waste containing water consists, via addition of CaO, in coating said waste in a dry matrix of Ca(OH)2. Compacting of a waste that does not contain water comprises mixing said waste with CaO then adding H2O in order to obtain the same dry matrix. Compacting, in particular carried out in a mold having nonstick walls, results in the formation of shaped products.